Feature Of The Week 4/8/12: Researchers at Osaka and Nagoya University Investigate The Use of Ultrahigh Resolution OCT for Imaging Protein Crystals

technique used for atomic-resolution protein-structure studies, and currently, robotic automation is often used to monitor and optimize crystallization conditions. However, it is often difficult to identify, by conventional light microscopy, crystals that can be used for X-ray crystallography when micro-size, needle, and salt crystals are also present in the crystallization medium.For the work reported herein, we demonstrated that micro-scale, non-invasive, three-dimensional (3D) cross-sectional, μm-resolution imaging of protein crystals could be achieved using optical coherence tomography (OCT).

A wideband, ultra-high resolution, OCT spectrum (supercontinuum) was generated using an ultrashort laser pulse and optical fibers as the light source. Given the high sensitivity of OCT and the enhanced scattering signal obtained when an agarose gel was included in the crystallization medium, protein crystals could be clearly observed.

Protein crystals could be differentiated from aggregates and amorphous material using this technique, even when such crystals could not be visualized by conventional light microscopic imaging. Because high-resolution imaging was achieved, needle-shaped crystals were also observed among the aggregates and amorphous material. We also discriminated between protein and salt crystals using the different OCT intensities of the crystals.

OCT system requires a very short time (5 sec/experiment) and is nondestructive. Therefore, when this strategy is used to visualize protein crystals, it may remove the remaining bottleneck that slows down high-throughput crystallization, and consequently lead to a new era in X-ray crystallography.